The use of enclosed metal molds is well known in the art of hollow molding, particularly in the rotational molding of vinyl plastisols and polyethylene. In these methods, the molding material is introduced into the mold through an opening in the surface thereof which is then tightly closed. This mold is then subjected to multi-axial rotation while being heated either by being placed in an oven or by being placed in contact with the flames of gas burners. The molding material is cured due to its contact with the hot interior walls of the mold. The fact that the mold is completely enclosed prevents the material from spilling out during rotation and excessively hot air from the heating medium is prevented from entering the mold and coming into contact with the yet uncured molding material and the heat accumulating in the air within the mold is thought to help effect the curing of the inside surface of the molding material.
Provisions are made in the above-described system to equalize the pressure inside the mold which is caused to change by the expansion and subsequent contraction of the air contained therein as the mold is heated and then cooled. Small vent holes are used for this purpose. These vents are necessarily small in size and act slowly thus necessitating that the closure of the mold be very tight and that the mold be of a generally heavy construction to withstand internal pressure and to support a clamping system to provide a tight closure. Air will otherwise pass through the closure and form a bubble or void in the molded part.
Parts molded in such molds are cooled only by the outward passage of heat through the mold walls. To hasten cooling, water is generally sprayed against the outside walls of these molds. It is known that the accelerated cooling of mold walls by contact with water causes articles made of certain rigid materials (such as high density polyethylene) to warp during the cooling phase.